Pins of this kind are used to fix an implant fitted in an opening in a bone. To do this, the implant is first pushed into the opening, in most cases a bore that has been formed from the outside. The pin is then driven in transversely thereto, thus passing transversely through the bore and through the implant pushed into the latter, as a result of which said implant is fixed in position. Since the pin extends transversely with respect to the longitudinal extent of the implant, the expression “cross pin” has become established.
A common application is in fixing an implant serving as a replacement for the cruciate ligaments of the knee.
For replacing the cruciate ligament, an operating technique has been developed in which the implant or graft, in most cases a tendon from the patient, is formed into a loop, and the cross pin is driven transversely through it in the area of the loop. It is also known for two loose ends of tendon sections to be sewn together. When moving the knee a strong tension acts on the tendon implant which is transferred to the pin.
The tendon loops round the rod-shaped body of the pin in a cross-sectional plane. The result of this is that the tensile load exerted by the tendon is input into the rod-shaped body on one side of its cross section and can be output on the opposite side to anchoring sites which are axially spaced apart from the input side. This anchoring site is the inner wall, geometrically speaking a surface line of the wall, of a bore in which the cross pin is received.
Such pins usually have a circular cross section so that the force output site lies along a surface line of the rod-shaped body lying opposite the loop or cross-section side around which the loop is guided. Additionally, the force input side is located in the central section of the pin, the force output sides are located at the opposite end sections of the pin resting in the bone.
In the event of loading, this has the effect that the force input by the tendon implant is output to the anchoring site in a more or less limited area on the end sections of the pin.
A study has established that this geometry can cause the cross pin to fracture.
In order to remedy this situation, it was attempted to introduce a second identical cross pin, with a likewise circular cross section, directly below the first cross pin in the direction of the force input.
A further study has now shown that even the placement of a second adjacent cross pin is unable to exclude the possibility of fracturing. On the contrary, if the first cross pin fractures under strong tensile loading, then the second cross pin also fractures thereafter. There is, as it were, a kind of domino effect.
It is therefore an object of the present invention is to remedy this situation and make available a pin of the aforementioned type for fixing an implant subjected to tensile load, which pin is able to withstand high tensile loads without fracturing.